Staggered axis labels

Monday, Tuesday,... oops, the X axis is beginning to look like a mess. When you
have one or more long labels on the axis, they can run together unless you
squash the font down to an unreadable size or stagger the labels. Using one of
the variables in the previous example, try this.

> upvar

First plot the data with no axes at all, then add the box and the ordinate. The
second call to axis() just makes the tick marks where we want them.
I've assigned the labels to a character vector to make it easier to see what's
going on here. mtext() then places the ten labels on alternate
lines, staggering them. You may want to add space for an extra line to the
bottom of the plot if you think it will look better.